The 31-year-old's previous deal was set to expire at the end of the season, leading to speculation linking him with a host of Premier League clubs.
And Jol has admitted he was not certain the centre-back would elect to remain with the Cottagers beyond the summer.
|20/1||Brede Hangeland is 20/1 with Bet365 to score first for Fulham against QPR
"I thought I could lose him - and he was a tower of strength this year, last year and before I came here. I always thought he wanted to stay, but you never know.
"He was on a free at the end of the year and it took a long time. But we did everything and in the end it was a happy outcome.
"He came to sign it on Thursday and it was a relief. He is one of the best players in the division, he is our skipper and a very nice man. So, we're very pleased to keep him."
QPR, who make the short journey to face Fulham on Monday night, were one of the clubs thought to be eyeing a move for Hangeland should he have failed to agree terms.
The Hoops tempted Bobby Zamora to Loftus Road in the summer but Jol insisted that he did not regret allowing the striker to leave.
"Bobby wanted to move," he claimed. "He thought he could help QPR and, if he's fit, he can help any team as he's a good striker."